Organic solvents are usually used in the preparation of magnetic recording materials, photographic light-sensitive materials, heat-sensitive sheets, etc.
Various drying methods have been proposed and are now in practical use for the prevention of pollution, recovery of organic solvents and so forth. For example, there is a method employed for magnetic recording materials in which a continuously running support, e.g., a plastic film, is continuously coated with a coating composition prepared by dispersing magnetic powder, such as ferromagnetic iron oxide, ferromagnetic chromium dioxide, and ferromagnetic alloys, and a binder in an organic solvent. If desired, the magnetic recording material may be subjected to various treatments such as magnetic field orientation. Thereafter, the coated support is sent to a drying room where it is continuously dried by blowing hot air thereonto while conveying it in an arch-like or straight form on a number of rolls provided in the drying room.
The various drying methods can be divided into two groups--i.e., a horizontal flow drying method and a vertical flow drying method. In accordance with the horizontal flow drying method, a coated support is conveyed in a drying room at a proper speed (as described hereinafter) and hot air is blown onto the coated surface of the support horizontally relative to the coated surface in either the same direction as or the opposite direction to the direction of the movement of the support. On the other hand, in the vertical flow drying method, hot air is blown vertically onto the coated surface of the support through holes or slits provided above a support-conveying unit in the drying room.
In each drying method, the solvent is vaporized during the drying procedure and thus there is the danger of an explosion when the gas concentration in the drying room reaches a certain level. It is therefore necessary to maintain the gas concentration below the lower critical limit of the solvent gas explosion concentration--e.g., about 25 to 33% to prevent an explosion. Hence, it is preferrable to feed a larger amount of air to maintain the gas concentration below the critical limit.
In accordance with the horizontal flow drying method, a good coated-surface can be obtained only when hot air is blown onto the support uniformly over the entire width of the support since, as described above, the hot air is blown directly onto the coated surface in either the same direction as or the opposite direction to the direction of the movement of the support.
In the early stages of the drying procedure, the coated surface still possesses fluidity. Thus, the coated surface is readily influenced by conditions such as the air-blowing speed. When the air-blowing speed is increased, the support beings to flutter and it becomes impossible to keep the support in a stable running condition.
In the vertical flow drying method, it is also necessary for the gas concentration to be maintained at a level below the lower critical limit of solvent gas explosion concentration (25 to 33%) for the sake of safety as previously described for the horizontal flow drying method. Further, it is desirable to increase the amount of air-feed (air-blowing speed). However, such an increase in the amount of air-feed leads to rapid solvent vaporization.
On the other hand, in order to obtain a uniform coated-surface, it is preferable to lower the solvent vaporization speed and to decrease the air-blowing speed since this reduces the influences caused by air-blowing, e.g., fluttering of the web. Hence, it is difficult to satisfy the two requirements of maintaining a low level of solvent gas concentration and a low solvent vaporization speed at the same time.
In accordance with conventional methods, it is difficult to produce a good coated-layer and increase productivity under safe conditions i.e., while preventing an explosion. The reasons for this are explained in detail below.
The temperature of dry hot air is usually from 50.degree. to 120.degree. C. (the boiling point of the primary solvent used is 130.degree. C. or lower). Within this temperature range, it is suitable for the speed of air to be blown onto the coated layer--i.e., the air-blowing speed, to be maintained within the range of from 1 to 5 m/sec (the maximum value: 10 m/sec). Further, it is required that the support-conveying speed be at least 40 m/min and the drying time be from 5 to 20 seconds. However, when the support-conveying speed is increased to, for example, 60 to 120 m/min (1 to 2 m/sec) or more in order to increase productivity, the amount of vaporization of the solvent increases. Therefore, if drying is performed without increasing the dry hot air-blowing speed (i.e., amount of dry hot air-feed), the solvent gas concentration increases. Hence, in performing drying by conventional methods, it is necessary to increase the hot air-blowing speed (i.e., amount of hot air-feed) to prevent an explosion. This makes it difficult to maintain the drying conditions required for producing a good coated-layer.